rr 5.7.0 released

[Robert O'Callahan]

https://github.com/rr-debugger/rr/releases/tag/5.7.0

I fixed a lot of intermittent bugs and wrote some code to build and test releases in AWS EC2 VMs. The instructions for preparing a release are here: https://github.com/rr-debugger/rr/tree/master/release-process

It was a lot of work but this should make it much easier to do releases in the future, and much easier for someone other than me to do releases if necessary.

The `test-system.py` script makes it easy to spin up a VM for rr debugging and testing, especially of intermittent rr bugs where running lots of instances of the test in parallel is helpful.

Google has generously let me spend a fair bit of time on this rr release.

Rob

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